1. Field of the Invention
The present invention relates to a control apparatus and method and a control unit which calculate a control input to a controlled object based on a value obtained by correcting a value calculated by a feedforward control method using a value calculated by a feedback control method.
2. Description of the Related Art
Conventionally, as a control apparatus for controlling the air-fuel ratio of a mixture supplied to an internal combustion engine, the present assignee has already proposed a control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-234550. This control apparatus is comprised of a LAF sensor, an oxygen concentration sensor, a state predictor, an onboard identifier, a sliding mode controller, and a target air-fuel ratio-calculating section. The LAF sensor and the oxygen concentration sensor are each for detecting a value indicative of the concentration of oxygen in exhaust gases flowing through an exhaust passage of the engine, that is, an air-fuel ratio, and are inserted into the exhaust passage at respective locations downstream of a collecting section thereof. Further, the engine is provided with a first catalytic device disposed in the exhaust passage at a location downstream of the collecting section, and a second catalytic device disposed on the downstream side of the first catalytic device. The LAF sensor is disposed on the upstream side of the first catalytic device, and the oxygen concentration sensor is disposed between the first catalytic device and the second catalytic device.
This control apparatus employs a discrete-time system model as a controlled object model to which is input the difference DKACT between an actual air-fuel ratio KACT detected by the LAF sensor and an air-fuel ratio reference value FLAFBASE (hereinafter referred to as the “air-fuel ratio difference DKACT”) and from which is output the difference DVO2 between an output VOUT of the oxygen concentration sensor and a predetermined target value VOUT_TARGET (hereinafter referred to as the “output difference DVO2”), and calculates a target actual air-fuel KCMD as a control input, as described hereinafter.
More specifically, the state predictor calculates a predicted value of the output difference DVO2 with a predetermined prediction algorithm based on the above-described controlled object model, and the onboard identifier identifies a model parameter of the controlled object model by an sequential least-squares method. Further, the sliding mode controller calculates an operation amount Usl based on the predicted value of the output difference and an identification value of the model parameter with a sliding mode control algorithm such that the output difference DVO2 converges to 0.
Furthermore, the target air-fuel ratio-calculating section calculates a target air-fuel ratio KCMD by adding the operation amount Usl to the air-fuel ratio reference value FLAFBASE, and a feedback correction coefficient-calculating section calculates a feedback correction coefficient KFB such that the air-fuel ratio difference DKACT converges to the target air-fuel ratio KCMD. Further, a basic injection amount-calculating section calculates a basic injection amount Tim by searching a map according to the rotational speed NE of the engine and an intake pressure PB. Furthermore, a demanded fuel injection amount Tcyl is calculated by multiplying the basic injection amount Tim by various correction coefficients.
Then, a fuel injection amount Tout is calculated by multiplying the demanded fuel injection amount Tcyl by the feedback correction coefficient KFB such that the actual air-fuel ratio KACT is caused to converge to the above-described target air-fuel ratio KCMD. As a consequence, the air-fuel ratio is controlled such that the output VOUT from the oxygen concentration sensor converges to the predetermined target value VOUT_TARGET. The predetermined target value VOUT_TARGET is set to such a value as will make it possible to obtain an excellent exhaust emission reduction rate of the catalytic device when the output VOUT from the oxygen concentration sensor takes the target value VOUT_TARGET.
When the above-described control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-234550 is attempted to be applied to an engine with small displacement, such as an engine for a motorcycle, it is envisaged to configure the control apparatus, as described below: In general, an engine with small displacement has a characteristic that an intake passage thereof is markedly shorter and a volume of an intake chamber thereof is considerably smaller than those of an engine with large displacement, so that intake pulsation and intake pressure pulsation in the intake passage of the engine with small displacement are larger than those in an intake passage of the engine with large displacement. Therefore, when the basic injection amount Tim is calculated according to the intake air amount or intake pressure, the reliability of a signal from an airflow meter or an intake pressure sensor is so low that the accuracy of the calculation of the basic injection amount Tim is lowered. To solve the problem, it is only required that as a map for use in calculating the basic injection amount Tim, a map associated with the opening TH of a throttle valve (hereinafter referred to as the “throttle valve opening TH”), detected by a throttle valve opening sensor, and the engine speed NE may be used in place of a map used in the control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-234550.
Further, if the LAF sensor of the control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-234550 is applied to the engine with small displacement, there arises not only the problem of increased costs due to the expensiveness of the LAF sensor, but also the problem of degraded fuel economy due to necessity of heating the LAF sensor by a heater so as to stabilize output therefrom. In view of the problems, it is necessary to omit the LAF sensor. In the case of thus omitting the LAF sensor, it is only required that the control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-234550 uses a discrete-time system model as a controlled object model to which is input the difference DKCMD between the target air-fuel ratio KCMD and the air-fuel ratio reference value FLAFBASE, and from which is output the difference DVO2 between the output VOUT of the oxygen concentration sensor and the predetermined target value VOUT_TARGET.
When the control apparatus for the engine with small displacement (hereinafter referred to as the “small-displacement control apparatus”) is configured as described above, although it is possible to attain the reduction of costs and the enhancement of fuel economy, when there occur three events: offset displacement, temperature drift, and sludge accumulation, described hereinafter, there is a fear that the basic injection amount Tim cannot be properly calculated. It should be noted that throughout the specification, “offset displacement” is intended to mean that the zero point position of the throttle valve sensor is displaced from a correct position thereof due to impact or mechanical play. Further, “temperature drift” is intended to mean that during high-load operation of the engine in a high temperature state, a signal from the throttle valve opening sensor drifts, whereby the throttle valve opening TH calculated based on the signal deviates from an actual value. Furthermore, “sludge accumulation” is intended to mean a state in which sludge is accumulated on the throttle valve and an inner wall of the intake passage around the throttle vale due to long-term use of the engine.
When the above-described offset displacement or temperature drift is caused, the relationship between an appropriate value (necessary value) of the basic injection amount Tim and the throttle valve opening TH deviates from the relationship between a map value and the throttle valve opening TH. It should be noted that in the following description of the specification, an error of the basic injection amount Tim calculated from a map with respect to the appropriate value is referred to as a “mapping error”. When such a mapping error us caused, in the above-described small-displacement control apparatus, air-fuel ratio feedback control is performed using the feedback correction coefficient KFB, so that when the engine is in a steady operating condition, it is possible to cause the output VOUT from the oxygen concentration sensor to converge to the predetermined target value VOUT_TARGET while compensating for the influence of the mapping error.
However, the feedback control method has a characteristic that it has lower responsiveness than that of the feedforward control method, and hence in the case of occurrence of the above-described mapping error, if the engine shifts from the steady operating condition to transient operating conditions, the influence of the mapping error cannot be properly compensated for, whereby the output VOUT from the oxygen concentration sensor deviates from the predetermined target value VOUT_TARGET. This results in the degraded accuracy of the air-fuel ratio control, causing increased exhaust emissions.
Further, when the sludge accumulation is caused, the intake air amount becomes lower than when the sludge accumulation is not caused, so that the relationship between the appropriate value of the basic injection amount Tim and the throttle valve opening TH and the engine speed NE deviates from the relationship between a map value and the throttle valve opening TH and the engine speed NE, causing a mapping error. As a consequence, as described above, when the engine is in transient operating conditions, the influence of the mapping error cannot be properly compensated for, which degrades the accuracy of the air-fuel ratio control, resulting in increased exhaust emissions.